1. Field of the Invention
The present invention relates to a grooved molding substrate (a substrate formed by a stamper), which has narrow grooves (on which pits are formed) and is used for optical disks, magneto-optical disks, hard disks (magnetic disks), and the like, and a method for manufacturing the same. Moreover, the present invention relates to a method for manufacturing a master substrate used for manufacturing the aforesaid grooved molding substrate and the stamper. Furthermore, the present invention relates to a recording medium using the grooved molding substrate, a memory device using the recording medium, and a computer using the memory device. Since the grooved molding substrate according to the present invention can be formed to have narrow grooves having the width of 0.23 μm or less, the recording density can be enhanced by applying the present invention to optical disks, magneto-optical disks, hard disks, and the like.
2. Discussion of the Related Art
Data recording media, such as optical disks, hard disks, and the like, are capable of recording large quantities of information. Such data recording media are commonly referred to as CD's (compact disks), LD's (laser disks), DVD's (digital video disks, or digital versatile disks), etc. These data recording media may contain music, movies, software, etc. Such media also are used as storage devices in computers. Demand for such recording media is expanding greatly. Indeed, it is anticipated that optical disk and hard disk usage will continue to expand because these are the major recording media of the multimedia age.
Optical disks are classified according to the existence or absence of a recording layer, and are further classified according to the type of recording layer. Optical disk types include:
(1) The read-only type (CD, LD, CD-ROM, photo-CD, DVD-ROM, read-only type MD, etc.);
(2) The write-once type (CD-R, DVD-R, DVD-WO, etc.); and
(3) The rewritable type capable of erasure followed by writing any number of times (magneto-optical disk, phase-change type disk, MD, CD-E, DVD-RAM, DVD-RW, etc.).
Moreover, the high density HD-DVD has also been proposed as a medium of the future.
Processes for manufacturing these optical disks begin with the molding of a raw material resin into a resin substrate. A raw material resin, for example, polycarbonate, acrylate resin, polystyrene, etc., is heated, melted or partially melted, and then is pressed using a stamper, thereby molding (manufacturing) a resin substrate. Typically, the molding method used is a pressure molding or injection molding method. The stamper forms fine concavities and protuberances which represent information copied upon the substrate surface. Other than resin molding, there is no such method for manufacturing large quantities of substrates that have minute concavities and protuberances in a short time period.
Types of pits and protuberances include:
(1) Pits that indicate a unit of information; and
(2) Guide grooves that are provided for tracking by the pickup head.
Generally, the manufacture of data recording media involves circular substrates provided with pits and grooves on the substrate surface in the pattern of concentric circular rings or in a spiral pattern. The region between grooves along the radial direction is called a “land.” Recording upon the lands occurs in the land recording method, or alternatively, recording occurs within the groove in the groove recording method.
In order to improve the recording density, the land/groove recording method was developed to record upon both the grooves and the lands. In this case, both grooves and lands are tracks, and the groove width Gw and the land width Lw are nearly equal. However, there are reasons for sometimes deliberately widening one or the other. Incident light enters the backside surface (flat smooth surface) of the substrate. In this case, the portion of the concavities and protuberances that is far from the backside surface is called “a land,” and the portion of the concavities and protuberances that is close to the backside surface is called “a groove.”
As the recording density increased, to meet the increased need for a larger storage capacity, the groove width, land width, and the pit width have decreased and their depth has increased. For example, the width has decreased from <1 μm to <0.3 μm and the depth has increased from >40 nm to >250 nm. As the width decreases and the depth increases (i.e., as the density becomes higher), molding of the resin substrate becomes increasingly difficult, and the yield of good products declines.
When manufacturing a hard disk, a magnetic recording layer is typically formed or deposited on an aluminum or glass substrate with recording carried out by a magnetic head. A reflection layer, a recording layer and a protection layer may then be formed on the resin substrate to produce the desired final product.
As the recording density increases, the recording layer becomes extremely flat and smooth. When the magnetic head becomes relatively still, the recording head and the recording layer adhere to one another and cannot separate. In order to avoid this phenomenon, a garage region (CSS region=contact stop and start) is provided. The surface of this garage region is deliberately finished with a rough texture using a laser so that surface adherence is prevented. Head tracking also becomes difficult as recording density increases. Therefore, it is proposed that a magnetic hard disk should be provided with grooves like an optical disk. Due to a demand for such roughness and grooves, resin substrates are proposed as a means to increase manufacturing productivity. Increased productivity results due to the formation of roughness and grooves in the substrate molding. In this case, material of the substrate is resin or low-melting glass.
Previously, molding tools were manufactured by the process described in Hunyar, U.S. Pat. No. 4,211,617, which corresponds to Japanese Patent publication Sho 59-16332, the disclosures of which are hereby incorporated by reference in their entirety. This related art (Hunyar) is explained with reference to FIGS. 9A to 11.
Generally, molding tools are manufactured using a glass substrate 3 that is polished with the precision of an optical surface. After the substrate 3 is cleaned, it is coated with a primer, for example, a silane-coupling agent. A photoresist 2 is then applied by spin coating and subjected to a pre-bake process. Positive-type photoresist 2, i.e., the type in which the region exposed to light is removed by development, is often used. This is because the surface roughness can be made smaller by the positive-type photoresist to have lower noise, which is advantageous. The following descriptions assume use of a positive-type photoresist.
Next, a laser beam recorder or a laser cutting machine is used to expose the photoresist 2 with a pattern of pits and/or grooves. The width of pits and grooves is generally determined by the laser spot diameter. In this case, in order to make the laser spot diameter as narrow as possible (i.e., to obtain a higher density), the laser beam is converged to the diffraction limit by a lens 1. On the other hand, the depth of the pits and grooves is generally determined by the thickness of the photoresist 2.
The case where a plurality of grooves exit in the pattern of concentric circular rings is explained in more detail. First, the photoresist 2 is illuminated by a predetermined exposure light along the first line O1 via lens 1 (FIG. 9A). The illumination is continuous when forming grooves, and is intermittent when forming pits. The illuminated area (exposed area) becomes the first groove of the molding substrate afterward. In this method, the spot diameter of the exposure light directly defines the line width of the “exposed area” (hereinafter, the “exposed area” may also be referred to as “exposure area”). In this case, the minimum spot diameter is defined by the diffraction limit of the exposure light, and it depends on the wavelength λ of the exposure light. Because the light intensity distribution in the light beam exhibits the Gaussian distribution, the intensity is the strongest at the center and becomes weaker at the periphery. Therefore, the effective spot diameter (diameter of the removed area of the exposed photoresist by development) becomes smaller than the value determined by the diffraction limit because of the sensitivity of the photoresist and the developing condition. When an exposure method called a “narrow pencil writing,” which uses only the center of the light beam by weakening the output of the light source, is used, the effective diameter can be made even smaller. In, the conventional method, the effective spot diameter φ determines the groove width of the resist pattern, and accordingly, determines the groove width Gw of the molding substrate. In, FIGS. 9A-9B, φ denotes the effective spot diameter.
At present, an argon laser light having the wavelength λ=351 nm is used for the exposure light. In this case, the minimum effective spot diameter φ is 0.23 μm. Accordingly, the minimum groove width Gw of the molding substrate that can be obtained is about 0.23 μm which is almost equal to φ (i.e., φ=groove width Gw).
When the groove width Gw needs to be large, the spot diameter is not made small to the diffraction limit, or exposure light in the out-of-focus condition is used. When the desired line width cannot be obtained by one exposure, another exposure similar to the first exposure can be performed repeatedly with the illuminating position moved by an appropriate distance. At any rate, the illuminating position is then moved from the first line O1 to the second line O2 separated by the distance corresponding to the sum of the groove width Gw and the land width Lw (which is parallel to the first line) (FIG. 9B).
After moving the exposure light to the position of the second line O2, the photoresist is illuminated (exposed). Generally, this process is repeated plural times successively regarding the second line O2 as the first line O1. This way, a plurality of the exposure areas 2e of concentric circular rings is obtained (FIG. 9C).
A resist pattern having grooves and pits on the substrate surface is obtained by developing the exposed photoresist. Following the development, the resist pattern may optionally undergo a 20 to 60-minute post-bake at 80-120° C. When such a post-bake is used, the resist pattern is then cooled down to a room temperature. This is shown in FIG. 10A.
The resist pattern in combination with the substrate 3 shown in FIG. 10A is called the master substrate or master 4. The master substrate 4 is equivalent to the replica 46 in FIG. 4 of Hunyar U.S. Pat. No. 4,211,617.
The master substrate 4 undergoes a metallization treatment to form a conductive layer on the surface. Generally such a treatment is carried out by sputtering (dry-type method), or by non-electrolytic plating (wet-type method). Following the metallization, a thick plating layer, such as nickel (Ni), is formed on the master substrate 4 by an electroforming method. The double layer structure that is made of the conductive layer and the Ni plating layer is referred to as the “father stamper” or just the “father” or “stamper.” This is shown in FIG. 10B. A free stamper 5 is obtained when the stamper 5 is peeled off from the master substrate 4. This is indicated in FIG. 10C. The stamper 5 is equivalent to mother member 52 in FIG. 6 of Hunyar U.S. Pat. No. 4,211,617.
Care must be taken during peeling since the stamper 5 is generally thin, approximately 200-300 μm in thickness. After peeling, the stamper 5 undergoes a solvent treatment, such as acetone treatment or the like, to remove the resist since a portion of the resist may remain on the stamper 5. The resist must be removed since the concavities and protuberances on the surface of the stamper would not otherwise be destroyed. Only a single stamper 5 is obtained from a single master substrate 4 since the resist pattern 2 is damaged during the peeling. The resulting stamper 5 has an extremely precise pattern of concavities-protuberances. Because the stamper 5 after peeling has a rather inaccurate outer dimension, a central hole is bored in the center of the stamper 5, and the unused portion of the outside perimeter is cut off. Before further processing, the concavity-protuberance surface (signal surface) is shielded with a protective coat. Thus, an annular shaped stamper 5 is obtained.
Then, a molding substrate is formed by using the stamper 5. A soft resin (or liquid resin) 6 is pressed against the stamper 5. This is shown in FIG. 10D. Accordingly, the concavities-protuberances of the stamper 5 are embossed on the resin. After cooling it down, the hardened or cured resin 6 is peeled off from the stamper 5 to form a molding substrate 6 shown in FIG. 11. The molding substrate 6 has the concavities-protuberances formed by grooves having the width Gw and the lands having the width Lw, which are disposed alternately. In order to manufacture the molding substrate, pressure molding or injection molding can be used. Generally, injection molding is used because of its high productivity.
As explained above, the width Gw of grooves (pits and dints, etc, are also generally referred to as “grooves”) is determined by the wavelength λ and the effective spot diameter φ. Therefore, the molding substrate having the groove width Gw narrower than φ cannot be obtained. Since argon laser (λ=351 nm) is used presently, the minimum groove width Gw that can be formed is about 0.23 μm (230 nm). If the wavelength λ can be made smaller, it may be possible to reduce the groove width Gw. However, a proper light source having a shorter wavelength than that of argon laser has not been available to date because there exists no appropriate laser having a shorter wavelength with continuous oscillation, or photoresist that has sensitivity to such a short wavelength λ (ultraviolet light) and that makes it possible to etch groove walls vertically. Accordingly, the groove width Gw has the shortest value of about 0.23 μm thus far. However, as the demand for higher recording densities increases, development of technology capable of forming a finer groove width Gw has been strongly desired.